Mucked around sacrificing coolant to the coolant gods in the overflow vent, after replacing all cooling system caps, so blanking caps for radiator and 16 PSI pressure cap to expansion tank, and cycling through from warm to cool and refill and repeat over and over (for any air trap removal), as per fault finding instructions from Brunswick, and guess what, no changes, overflow tank still blowing out coolant and coolant does not get hotter than 60 degrees by the laser temp gun on the two tanks.
So pressurization coming from somewhere rightdeptie being told it is unlikely to be a head gasket or a cracked head. So the discussions will continue.
Evidently BD has identified a shop in Blackheath (in the blue mountains) that is happy to work on these 6.5 chevs. so looks like we will be dropping in there eventually.